A long-standing hypothesis holds that memory formation involves structural changes in synapses. Indirect evidence shows that dendrites, dendritic spines, and axons in the central nervous system can grow and retract under a variety of different conditions. Recent advances in imaging technology, such as multiphoton microscopy, have made it possible to directly image morphological changes in vitro and in vivo, but it has proven difficult to link these changes to the actual process of learning. This is due to the inaccessibility of brain areas involved in the learning process (such as the hippocampus) and to difficulties in identifying the synapses that are involved in learning. The overall aim of this proposal is to use the mouse olfactory system to determine whether changes in dendritic architecture accompany the formation of long-term memories. The formation of olfactory memories requires the olfactory bulb, whose location makes it very accessible for high-resolution imaging studies. Using genetically engineered mice in which specific neuronal populations are labeled with fluorescent markers, multiphoton imaging of neuronal dendrites over hours, days, and weeks will be used to determine the stability of dendrites in the adult brain. Animals will then learn an odorant discrimination task, and imaging will be used to investigate the extent to which learning the task results in dendritic alterations. To begin to address possible mechanisms involved in dendritic remodeling, a preparation will be developed in which learning can be induced while animals are anesthetized, which will allow real-time observations and manipulations of circuits in the bulb. These experiments will determine whether structural changes are a requisite for long-term memories. Understanding the cellular changes that accompany the formation of memories is critical for understanding how normal memory formation takes place, and in uncovering mechanisms that can be targeted for intervention in the many disorders of memory, both in normal aging and in pathological conditions such as Alzheimer's disease. [unreadable] [unreadable]